The long-term goal of the proposed research is to understand the molecular mechanisms of synaptic plasticity. Heterotrimeric G proteins have been implicated as regulators of synaptic plasticity in various organisms, but the mechanisms by which they regulate synapse strength are not well understood. This application aims to characterize the pathways by which Gq acts as a positive regulator of synaptic transmission in the nematode C. elegans. A genetic screen for suppressors of an activated Gqa mutant led to the identification of new components of pathways acting downstream of Gqa, including a guanine nucleotide exchange factor for a small GTPase, and a novel RUN-domain protein that is hypothesized to function as an effector of a small GTPase. Aim 1 will identify the small GTPase and characterize its interactions with the Gqa pathway, in particular determining whether the small GTPase and the RUN-domain protein physically interact. Genetic interactions will be determined by performing behavioral and pharmacological assays of mutant animals. Biochemical interactions will be determined by in vitro binding assays using GST-pulldowns. Aim 2 will determine the mechanisms by which Gqa and the small GTPase pathway regulate synaptic release. Electron microscopy and synaptic electrophysiology will be used to characterize the effects of altered pathway activity on vesicle docking/priming and the probability of vesicle release. Learning these techniques is the major training goal of the mentored phase of this grant. Aims 3 and 4 will identify more molecules acting downstream of Gqa and determine their mechanisms of action using the methods of Aims 1 and 2. Aims 1 and 2 will be completed during the mentored phase, Aims 3 and 4 during the independent phase. These studies will be a major step forward in defining the molecular pathways of Gq action in modulating synaptic strength. Many neuromodulators linked to human behavioral disorders act through G protein-coupled pathways. Understanding the pathways downstream of these neuromodulators will lead to a better understanding of the mechanism of these diseases and facilitate the design of better drug treatments. Because these pathways are modulatory rather than essential for neurotransmission, humans with mutations in these pathways would be expected to be viable, but mentally ill. Thus, the new genes identified in this work will be good candidates for genes linked to mental health disease in humans. Relevance: Human nervous system disorders such as schizophrenia, depression and attention deficit/ hyperactivity disorder are linked to abnormal levels of brain chemicals that affect the strength of signaling between different brain cells. This application aims to understand how such chemicals affect communication between cells in the nervous system so that better drugs can be designed to treat these disorders.